With 4 players online I get a lot of "Can't keep up messages" if someone decides to go by minecart or horse.
This happens if the players are slightly spread out in the world.
Environment
Server specs:
Linux 3.16.0 #1 SMP PREEMPT
Intel(R) Core(TM)2 CPU 6700 @ 2.66GHz
Java(TM) SE Runtime Environment (build 1.8.0_20-b26)
Java HotSpot(TM) 64-Bit Server VM (build 25.20-b23, mixed mode)
This is still an issue in 1.8.1, it's better but not as good as the 1.7.x versions.
Server can't keep up and it misses ticks because of that.